The viscosity, ultrasonic velocity, refractive index, and density of cellulose acetate/carboxylated poly(vinyl chloride) (CA/C-PVC) blends in cyclohexanone, a common solvent, were measured at 30 degreesC. After casting films of the blend for a 50:50 composition, polarizing micrographs were taken. Using the viscosity data, the interaction parameters were computed by employing two different equations. These values indicated that the CA/C-PVC blend is miscible. This was confirmed by ultrasonic, refractive index, density, and polarizing microscopic methods.